A media watchdog group on Friday denounced the maker of the hugely popular video game "Grand Theft Auto: San Andreas" over graphic sexual content that allegedly exists in the game and can be unlocked with an Internet download.

The game's plot is already objectionable to many people: Its main character carjacks for fun and profit and picks up women along the way.

But some say its content becomes sexually explicit if players download and install a modification to the game -- one of many so-called "mods" available on Web sites maintained by video game enthusiasts.

"While San Andreas is already full of violent behavior and sexual themes, the pornographic sex scenes push it over the edge," said David Walsh, founder of The Minneapolis-based National Institute on the Media and the Family, which issued a "nationwide parental alert" Friday.

The controversy has prompted an investigation by the Entertainment Software Ratings Board, which determines the rating on every video game sold. Rockstar Games issued a statement Friday, confirming the investigation and avoiding comment on whether its programmers created the sex scenes in the first place.

"We also feel confident that the investigation will uphold the original rating of the game, as the work of the mod community is beyond the scope of either publishers or the ESRB," the company said.

The mod's author -- Patrick Wildenborg, 36, of Deventer, Netherlands -- told The Associated Press on Friday that his code merely unlocks content that is already included in the code of each off-the-shelf game.

"If Rockstar Games denies that, then they're lying and I will be able to prove that," Wildenborg wrote in an e-mail. "My mod does not introduce anything to the game. All the content that is shown was already present on the DVD."